Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data communication. In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
In a wireless communication system, one carrier is considered in general even if a bandwidth is differently set between an uplink and a downlink. In 3rd generation partnership project (3GPP) long term evolution (LTE), one carrier constitutes each of the uplinks and the downlinks on the basis of a single carrier, and the bandwidth of the uplink is symmetrical to the bandwidth of the downlink. However, except for some areas of the world, it is not easy to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a spectrum aggregation technique is being developed to obtain the same effect as when a band of a logically wide bandwidth is used by physically aggregating a plurality of bands in a frequency domain. The spectrum aggregation includes a technique for supporting a system bandwidth of 100 mega Hertz (MHz) by using multiple carriers even if, for example, the 3GPP LTE supports a bandwidth of up to 20 MHz, and a technique for allocating an asymmetric bandwidth between the uplink and the downlink.
A mobile station (MS) may be in a radio resource control (RRC) connected mode or in an idle mode. When the MS is in the RRC connected mode, a radio link between the MS and the BS is connected. However, when a channel state deteriorates, out-of-synchronization of a radio link may be generated from a physical layer of the MS. When the out-of-synchronization continues to occur more than a certain number of times, the MS declares a radio link failure (RLF). When the RFL is declared, the MS reselects a cell with good quality and performs RRC connection reestablishment. In reestablishing the RRC connection, the MS, supporting a multi-carrier aggregation, is supposed to reestablish a cell regardless of a configuration of carrier aggregation. Thus, if the reselected cell does not belong to the carrier aggregation, a problem may arise in that the service transmitted and received by the MS is deferred or interrupted due to the RLF. Therefore, a method of configuring a radio connection in a multi-carrier system is required.